The present invention relates to a leakage inspection method for successively inspecting various products under production process, which are required to allow no fluid leakage in use or to keep any fluid leakage within a certain range, to determine the acceptability of the inspected products.
There are various products which are required to be airtight, examples being containers for containing liquids, parts in hydraulic circuits such as automotive brakes, radiators, engine cylinders, products for use with a gas, and wristwatches. Therefore, it has been required in the art to confirm the desired airtightness of the products.
Different methods are known which test whether an object is airtight. One method includes the steps of sealing air in the object, immersing the object in water, and visually checking whether air bubbles are produced from the object. Another method uses a water-column manometer. These known methods have a low detecting sensitivity and difficulty in detecting a slight leakage. Since the object has to be immersed in water or water is poured into the object, the object must be dried at a later time. Another disadvantage is that it is difficult to fully automatize these inspection methods.
There has been proposed a leakage inspection apparatus in which a positive or negative air pressure is introduced into an object to be tested, any variation in the air pressure is detected as an electric signal, and a change in the electric signal is compared with a preset level for determining whether there is a leakage or not and the extent of a leakage, if any. The proposed leakage inspection apparatus imposes no adverse effect on the object as the object is not immersed in water for inspection and hence is not caused to be wet. Since the acceptability of the object tested is determined by comparing electric signals, the apparatus can be fully automatized.
The leakage inspection apparatus of the type described is generally composed of a pnuematic circuit and an electric circuit. The pneumatic circuit includes an air pressure source for generating a certain positive or negative air pressure, a solenoid-operated valve for applying the air pressure from the air pressure source to the object and shutting off the air pressure, and a pressure sensor for detecting a change in the air pressure supplied to the object. The electric circuit comprises an amplifier for amplifying an electric signal issued from the pressure sensor, a discriminator means for comparing a change in the amplified electric signal from the amplifier with a preset level to determine the acceptability of the object, and a controller for sequentially controlling the various parts to repeat a test cycle composed of a pressurizing period in which the solenoid-operated valve is controlled to introduce the air pressure into the object, a balancing period in which the air pressure introduced in the object is stabilized, an inspecting period in which the air pressure in the object is checked for any variation after the time when the balancing period is over, an exhausting period in which air is exhausted from the object upon completion of the inspection, and a rest period in which the inspected object is replaced with another object after air has been exhausted.
The inspection process for applying an air pressure to the object and picking up a change in the air pressure as an electric signal is advantageous in that no water is brought into contact with the object, but has the following disadvantage:
Since air is a gas, it expands and contracts dependent on the temperature of the object, thus changing the air pressure. Therefore, even when there is no leakage under a constant air pressure introduced in the object, the air pressure tends to vary due to thermal expansion or contraction of air, and such a pressure change is responsible for a false determination that there is a leakage.
For example, when an ambient temperature rises gradually, the temperature of air in the object and the temperature of the object itself vary as the ambient temperature changes, with the result that the air pressure in the object will vary due to the variation of the ambient temperature, thereby changing a zero point. A conventional solution to this problem has been to change a preset reference level for determining the presence of a leakage through human intervention as the ambient temperature changes.
However, manually changing the preset reference level requires an operator to attend the inspection apparatus, resulting in an obstacle to automatization.
To cope with the prior problem, the applicant proposed a leakage inspection apparatus as disclosed in Japanese Patent Laid-Open Publication No. 58-50445. The disclosed leakage inspection apparatus has a data storage means which stores data items on a plurality of pressure measurements. When one of such data items is stored, the oldest pressure measurement data item is erased, so that a certain number of latest data items are always stored in the storage means. The latest data items are read out, and an average of such data items is computed. By repeating such a process, a moving average can be obtained which follows a variation in the pressure measurement data items, and is used as a corrective value. A variation of the zero point due to a change in the ambient temperature can therefore be removed by subtracting the moving average or corrective value from a pressure value as currently measured. A similar arrangement is also disclosed in applicant's U.S. application Ser. No. 674, 908 filed Nov. 9, 1984.
With the above inspection apparatus, where the ambient temperature changes, a corresponding change in the air pressure in the object is stored as a data item, an average of changes of air pressures in a plurality of objects inspected is determined, and such an average is employed as a corrective value. In the event of a gradual change in the ambient temperature, the average of the pressure data items is also varied, with the result that the zero point can be corrected properly at all times to remove undesired influences due to the change of the ambient temperature. The above function is referred to by the applicant as an automatic zero-point correcting function.
With this automatic zero-point correcting function, the zero point can automatically be corrected properly without human intervention for leakage inspection of high accuracy.
There are certain occasions in which objects of different shapes should be inspected for leakage. Where objects are of different shapes, deformations caused by applying an air pressure to the objects vary from object shape to object shape, and pressure variations generally differ from object type to object type. Therefore, when an object of different shape is to be inspected, pressure data items stored in the storage means should all be rewritten and a new moving average should be determined.
The storage means normally stores 5 to 10 measured pressure values, and sometimes stores about 100 measured pressure values. Accordingly, it is impossible for one inspection apparatus to inspect a variety of objects in a production line along which various differently shaped objects flow successively.
Where an object is found to allow a large leakage by the leakage inspection apparatus, the air pressure is greatly changed immediately after the air pressure is applied to the object. When this happens, the pressure change already exceeds the preset level for the balancing period in the sequence of the pressurizing period, the balancing period, the inspecting period, the exhausting period, and the rest period. If the pressure change exceeds the preset level in the balancing period, then the conventional apparatus determines that the object suffers from a large leakage. Such determination is performed before the foregoing corrective action is effected.
In determining whether there is such a large leakage, if the pressure sensor and the amplifier for amplifying an output of the pressure sensor electrically undergo a zero point change in excess of the preset level, then the apparatus will determine that there is a large leakage when in fact there is no such a large leakage allowed by the object.